1. Field of the Invention
This invention relates to the field of data communications. More specifically, it relates to the field of bidirectional data communications channels and to the field of bidirectional data communications channels for Fast Ethernet Local Area Network (LAN) communications and the IEEE standard for physical media connections referred to as 100BaseT4.
2. The Prior Art
For Fast Ethernet LAN communications (i.e. 100 MBit/sec), there are currently three approved IEEE standards for physical media connections. These three standards are commonly referred to as 100BaseTX (TX), 100BaseFX (FX), and 100BaseT4 (T4).
TX specifies that the transmission media is to be two pairs of unshielded twisted pair (UTP) cable of Category 5 or better. Of the two pairs, one pair is used as the transmission media for a dedicated transmit channel and the other pair is used as the transmission media for a dedicated receive channel. FX specifies that the transmission media is to be a fiber-optic cable. T4 specifies that the transmission media is to be four pairs of UTP cable of Category 3 or better.
As used throughout this discussion, a "T4 device" is any device that is designed to communicate using the 100baseT4 standard. Referring initially to FIG. 1, a first T4 device 10 and a second T4 device 12 are shown hooked together according to the 100BaseT4 communication standard. As noted above, the 100baseT4 standard specifies that the transmission media is to be four pairs of unshielded twisted pair cable. Each of these pairs are shown schematically in FIG. 1 as lines 14, 16, 18, and 20. How each of the four pairs 14, 16, 18, and 20 is utilized depends on the perspective of the T4 device.
From the point of view of the first T4 device 10, the first pair 14 is the transmission media for a dedicated "transmit" channel and the second pair 16 is the transmission media for a dedicated "recieve" channel. From the point of view of the second T4 device 12 however, the first pair 14 is the transmission media for a dedicated "recieve" channel and the second pair 16 is the transmission media for a dedicated "transmit" channel. From either perspective, the third and fourth pairs 18 and 20 are each used as the transmission media for channels that both transmit and receive. The third and fourth channels are therefore "bidirectional" channels. Each of these bidirectional channels can be configured to be either a transmit channel or a receive channel, but neither can be configured for both transmit and receive simultaneously.
When the first T4 device 10 is transmitting, data signals are transmitted from the first T4 device 10 to the second T4 device 12 on the dedicated transmit channel of the first T4 device 10 and each of the two bidirectional channels while the dedicated receive channel of the first T4 device 10 is used by the first T4 device 10 for sensing a collision. A collision occurs when any data signals are received by the first T4 device 10 on its dedicated receive channel while data is being transmitted on any of the other three channels.
When the first T4 device 10 is receiving, data signals are received by the first T4 device 10 from the second T4 device 12 on the dedicated receive channel of the first T4 device 10 and each of the two bidirectional channels while the transmit channel of the first T4 device 10 is used to send a collision signal if required.
Even though a bidirectional channel can be configured as either a transmitter or a receiver, the impedance looking into the bidirectional channel of the T4 device must always be matched to the characteristic impedance of the transmission media that is attached to it. This is true whether the T4 device is transmitting or receiving.
There are two known configurations of prior-art bidirectional communications interfaces being used for T4 bidirectional channels. These are shown in FIGS. 2 and 3.
Referring now to FIG. 2, the first prior-art bidirectional current source type communications interface 30 is shown. The first interface 30 includes an integrated circuit 32 that contains a transmitter 34 and a receiver 36. The output of transmitter 34 is connected to a pair of output pins 38 and 40. The input of receiver 36 is connected to a pair of input pins 42 and 44.
The first interface 30 has two paths from the four pins of the integrated circuit 32 identified above that are combined into a single path on two lines at connector 46. A transmit path has a first end at output pins 38 and 40 and a receive path has a first end at input pins 42 and 44. The transmit and receive paths are combined into a single bidirectional path such that both the transmit path and the receive path have a common second end at pins 48 and 50 of the connector 46. Transmission media 52 is connected to pins 48 and 50 of the connector 46.
According to the 100BaseT4 standard, the connector 46 is a RJ-45 connector and the transmission media 52 is a UTP cable that exhibits a characteristic impedance of 100 .OMEGA..
In operation, a transmit data signal is generated by the transmitter 34 and enters the transmit path at the integrated circuit 32 at output pins 38 and 40. It then passes through a transformer, 54. Finally, it exits the transmit path and passes on to the transmission media 52 through pins 48 and 50 of the connector 46. The transformer 54 has a common first winding 54a that is coupled to both a second winding 54b and a third winding 54c. The second winding 54b has a center tap 56 which is connected to a fixed voltage potential VCC 58. A first resistor 60 is connected between pin 38 of the integrated circuit 32 and VCC 58. A second resistor 62 is connected between pin 40 of the integrated circuit 32 and VCC 58.
A receive data signal is generated by a remote device (not shown) on the transmission media 52 and enters the receive path at pins 48 and 50 of the connector 46. It then passes through transformer 54. Finally, it exits the receive path and passes into the integrated circuit 32 to receiver 36 at input pins 42 and 44.
In circuits which are integrated onto a semiconductor chip, it is desirable to provide the same functions using fewer pins or to provide more functions using the same number of pins. The advantage of using fewer pins is that it reduces the size of the chip and the expense of packaging the chip. For example, a design that uses only two pins to perform a function is superior to a design that uses four pins to perform the same function. Similarly, in circuits requiring magnetics, it is desirable to provide the same function using fewer or smaller magnetics. The advantage of using fewer or smaller magnetics is that it takes up less space and is less expensive. For example, a design that uses only one two winding transformer to perform a function is superior to a design that uses a pair of two winding transformers or a three winding transformer to perform the same function.
Also, in electrical circuits, it is desirable to provide the same function using fewer separate paths. The advantage of using fewer circuit paths is also that it takes up less space and is less expensive. For example, a design that uses only one path to perform a function is superior to a design that uses two paths to perform the same function.
The first bidirectional communications interface 30 is less than ideal by virtue of the fact that it uses two pairs of input/output (I/O) pins, output pins 38 and 40 and input pins 42 and 44, at the integrated circuit 32 it uses a three winding transformer 54, and it uses two paths for transmit and receive. These each add to the cost and the size of the T4 device.
Referring now to FIG. 3, a second prior-art bidirectional voltage source type communications interface 70 is shown. The second interface 70 includes an integrated circuit 72 that contains a transmitter 74 and a receiver 76. The output of transmitter 74 is connected to a pair of output pins 78 and 80. The input of receiver 76 is connected to a pair of input pins 82 and 84.
The second interface 70 also has two paths from the four pins of the integrated circuit 72 identified above that are combined into a single path on two lines at a connector 86. A transmit path has a first end at output pins 78 and 80 and a receive path has a first end at input pins 82 and 84. The transmit and receive paths are combined into a single bidirectional path such that both the transmit path and the receive path have a common second end at pins 88 and 90 of the connector 86. Transmission media 92 is connected to pins 88 and 90 of the connector 86.
A transmit data signal is generated by the transmitter 74 and enters the transmit path at the integrated circuit 72 at output pins 78 and 80. It then passes through series resistors 94 and 96 and a transformer 98. Finally, it exits the transmit path and passes on to the transmission media 92 through pins 88 and 90 of the connector 86.
A receive data signal is generated by a remote device (not shown) on the transmission media 92 and enters the receive path at pins 88 and 90 of the connector 86. It passes through the transformer 98. Finally, it exits the receive path and passes into the integrated circuit 72 to receiver 76 at input pins 82 and 84.
The second prior-art bidirectional communications interface 70 is also less than ideal by virtue of the fact that it uses two pairs of I/O pins, output pins 78 and 80 and input pins 82 and 84, at the integrated circuit 72 and it uses two paths for transmit and receive. These each add to the cost and the size of the T4 device.